The cements used in oil-wells are classified in various classes according to the depth, the temperature and the pressure at which they are used, and also as a function of the compressive mechanical strength and of the resistance to the action of sulfates which are whished. This is the reason why the American Petroleum Institute (API) has set up a classification of the Portland cements from class A to class J with sub-classes MSR if a moderate sulfate resistance is wished and HSR if a high sulfate resistance is wished.
At present, it is necessary to stock on the working site various categories of cement, and use must be made of expensive additives, and especially of retarders of set for the high-temperature areas. Some of these retarders cause irregular setting times of the cements and perturb their hardening. It is furthermore observed that the mechanical properties of all the cements which are presently available fall off rapidly in course of time when the hardened cement is subjected to temperatures higher than 110.degree. C. An exception is a cement belonging to class F of the API, since it already contains 30% silica. The addition of silica limits in fact the fall off at high temperatures of the mechanical properties, but this addition jeopardizes the stability and the homogeneity of the cement formulas, since the siliceous products are not subjected to strict rules concerning the mesh size, the specific surface and the purity grade: this addition involves furthermore an increased consumption of additives, in particular of liquifiers, and makes it difficult to obtain dense cements.